On Fri, Apr 22, 2022 at 11:19:05AM +1000, Dave Chinner wrote: > On Tue, Apr 19, 2022 at 09:42:30AM -0700, Roman Gushchin wrote: > > On Tue, Apr 19, 2022 at 02:56:06PM +0200, David Hildenbrand wrote: > > > On 16.04.22 02:41, Roman Gushchin wrote: > > > > add_nr_deferred() is often called with next_deferred equal to 0. > > > > For instance, it's happening under low memory pressure for any > > > > shrinkers with a low number of cached objects. A corresponding trace > > > > looks like: > > > > <...>-619914 [005] .... 467456.345160: mm_shrink_slab_end: \ > > > > super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 \ > > > > unused scan count 0 new scan count 0 total_scan 0 \ > > > > last shrinker return val 0 > > > > > > > > <...>-619914 [005] .... 467456.345371: mm_shrink_slab_end: \ > > > > super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 \ > > > > unused scan count 0 new scan count 0 total_scan 0 \ > > > > last shrinker return val 0 > > > > > > > > <...>-619914 [005] .... 467456.345380: mm_shrink_slab_end: \ > > > > super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 unused \ > > > > scan count 0 new scan count 0 total_scan 0 \ > > > > last shrinker return val 0 > > > > > > > > This lead to unnecessary checks and atomic operations, which can be > > > > avoided by checking next_deferred for not being zero before calling > > > > add_nr_deferred(). In this case the mm_shrink_slab_end trace point > > > > will get a potentially slightly outdated "new scan count" value, but > > > > it's totally fine. > > > > > > Sufficient improvement to justify added complexity for anybody reading > > > that code? > > > > I don't have any numbers and really doubt the difference is significant, > > Never been able to measure it myself. > > HwoeverI'd much prefer the tracepoint output stays accurate - I've had to > post-process and/or graph the shrinker progress as reported by the > start/end tracpoints to find problems in the algorithms in the past. > That's why there is the additional complexity in the code to make > sure the coutners are accurate in the first place. Sure, no problems. Andrew, can you, please, drop this patch? Thanks!
